This invention relates generally to the combustion of liquid or slurried carbonaceous material fuels, where the term "carbonaceous material" as used herein means coal (including anthracite), coke (including petroleum coke), char, and oil (natural or synthetic). This invention is particularly directed to an arrangement for directing a coal-water or petroleum-coke-water mixture fuel into a combustion chamber for combustion therein.
A bulk liquid fuel is ideally subjected to an atomization process prior to its combustion for ensuring stable and efficient burning. During this atomization process, the liquid fuel is preferably broken up into the smallest possible droplet size to expedite its preheating and to ensure more probable and timely ignition of the fuel. In the case of a coal-water mixture fuel, atomization of the fuel preferably subdivides the bulk fuel to roughly the size of the coal particles suspended within the slurry to allow preheating and ignition to begin immediately for at least some of the coal particles. If the fuel slurry is not subdivided to this extent, the water tends to encase the coal particles and acts as a thermal insulator resulting in ignition delay and burnout of the fuel. Also, the coal particles tend to agglomerate.
In general, prior art fuel burner arrangements for use with coal-water slurry fuels have sufferend from various limitations. For example, the build-up of unburned carbon residue adjacent to the exit of the fuel delivery tube reduces the fuel flow within the system and degrades the combustion characteristics of the injected fuel. In addition, prior approaches have met with only limited success in attempting to atomize the fuel slurry to particle sizes on the order of the coal particles suspended within the slurry for preheating the coal and improving its combustion ignition. Failure to fully atomize the fuel slurry leads to the deposit of large collections of fuel particles within the combustion chamber and results not only in inefficient combustion but also leads to combustion chamber fouling as its operating characteristics are degraded by the unburned fuel, or slag, which collects therein. Examples of prior art fuel burners can be found in U.S. Pat. Nos. 3,280,882 to Hemker, 3,299,841 to Hemker et al and 4,223,615 to Breen et al as well as in "Coal Oil Mixture Utilization in Small Industrial and Commercial Boilers" by Savage et. al, 4th Int. Symposium on Coal Slurry Combustion, Vol. 2 (1982).
Anthracite coal is a particularly attractive fuel source in some locations because it comprises the largest coal reserves in these locations and possesses a low sulfur content eliminating the environmental requirement in many cases of sulfur dioxide scrubbers in treating the combustion exhaust gases. The close proximity of large deposits of anthracite coal in Eastern Pennsylvania to the New England and Mid-Atlantic industrial regions offers capital and operating cost advantages of anthracite over bituminous coal and further enhances the commercial attractiveness of this source of energy. Anthracite-water slurries thus have the potential to become competitive with bituminous coal and No. 6 fuel oil in both the new and retrofit boiler application markets.
An even more attractive fuel source is petroleum coke. Approximately 17 million tons of petroleum coke are produced annually as a by-produce of oil refinery operations. About half of the annual production (the higher-quality, low-sulfur coke) is used to manufacture industrial products, such as carbon electrodes. However, recently the petroleum industry has been forced to use more lower-grade, high-sulfur crudes, which has led to the production of increasing quantities of undesirable low- and medium-quality coke. The use of these cokes in fuel mixtures with water represents a potential means of disposal while reducing comsumption of fuel oil. The ash content of coke is usually less than 0.5 percent, which adds to its attractiveness as a slurry fuel.
However, anthracite coal and petroleum coke in slurry forms are not without limitations as energy sources. For example, anthracite-water and petroleum-coke-water slurries are difficult to burn without a support fuel and without oxygen enrichment of the combustion air because of their low volatile matter content, which results in long particle burnout times. In addition, while all coal-water slurries are difficult to atomize because they tend to plug passages in nozzles more than coal-oil slurries or fuel oil, anthracite-coal slurries are particularly difficult to atomize because of the hardness of the anthracite and the increased difficulty of reducing the liquid fuel to the smallest possible droplet size. The generally larger anthracite particle sized arising from its relative hardness also result in an increased tendency for it to collect within and clog the fuel flow paths in a fuel burner. Finally, prior art fuel burners have not been adequately isolated from the high temperatures of the combustion chamber and have thus generally operated at reduced efficiencies, have tended to plug up, and have been subject to increased wear exhibiting reduced reliability, particularly in the case of those burners having moving parts. These comments apply especially to small burners.
The present invention is intended to overcome the limitations of the prior art providing a fuel burner particularly adapted for the combustion of carbonaceous material-water slurries which includes a stationary high pressure tip-emulsion atomizer which directs a uniform fuel into a shearing air flow as the carbonaceous material-water slurry is directed into a combustion chamber, inhibits the collection of unburned fuel upon and within the atomizer, reduces the slurry to a collection of fine particles upon discharge into the combustion chamber, and regulates the operating temperature of the burner as well as primary air flow about the burner and into the combustion chamber for improved combustion efficiency, no atomizer plugging and enhanced flame stability.